US2645737A

US2645737A - Traveling wave tube

Info

Publication number: US2645737A
Application number: US102317A
Authority: US
Inventors: Lester M Field
Original assignee: Leland Stanford Junior University
Current assignee: Leland Stanford Junior University
Priority date: 1949-06-30
Filing date: 1949-06-30
Publication date: 1953-07-14
Anticipated expiration: 1970-07-14
Also published as: FR1022897A

Description

July 14, 1953 L, HELD 2,645,737

TRAVELING WAVE TUBE Filed June 50, 1949 Q INVENTOR LESTER M. F/ao f y/1W ATTORNEY Patented July 14, 1953 TRAVELING WAVE TUBE Application June 30, 1949, Serial No. 102,317

Claims. 1

This invention relates to electron discharge devices, and more particularly to improvements in travelling wave tubes of the type wherein amplification of wave energy is efiected by interaction between a travelling wave and a stream of electrons moving with the wave.

For successful operation of such tubes, it is necessary that the electrons and the wave to be amplified travel at approximately the same velocity. The electron velocity depends upon the magnitude of the voltage used to accelerate the stream. Owing to the increase in mass with velocity, the additional voltage required to produce a given additional electron velicity will increase more and more rapidly as the velocity of light is approached. For this reason it is impracticable at present in tubes of the described type to use electron velocities much greater than one quarter the velocity of light. Consequently some means is required for propagating the wave at a corresponding low velocity with respect to that of light.

A typical prior art structure for this purpose comprises a helix which is supplied at one end with the input energy in such manner as to carry a slowly travelling wave. The electron stream is directed along the longitudinal axis of the helix in the form of a beam, and amplified energy appears at the output end of the helix.

The cross sectional area of the electron beam is limited by the size of the helix, which in turn depends more or less upon the wavelength of the energy to be amplified. The beam current is the product of the cross sectional area and the electron density, and may be increased by increasing the density. However, electrons repel each other, so that a, dense beam tends-to spread out. Thus the total beam current, and hence the power handling capability, is limited in the helix type tube. Moreover, the helix ordinarily consists of many turns of relatively fine wire, and must be supported in such manner as to maintain its shape and pitch.

One of the principal objects of the present invention is to provide travelling wave tubes having improved power capability and simpler and inherently more rugged structure than corresponding prior art devices.

Another object is to provide an improved type of slow wave propagating structures for travelling wave tubes.

A further object of the invention is to provide efficient transition or transformer means for coupling conventional coaxial transmission lines to slow Wave propagating structures,

Another important object of the invention is to provide travelling wave tubes using an electron stream of relatively large cross sectional area, whereby a large beam current may be obtained with relatively low electron density.

The invention will be described with reference to the accompanying drawings, wherein:

Fig. 1 is a longitudinal section of a part of a slow wave propagating structure according to the present invention,

Fig. 2 is a family of graphs showing how the propagation velocity in a device like that of Fig. l varies as a function of frequency with various dimensional relationships,

Fig. 3 is a longitudinal section of a travelling wave tube constructed in accordance with the principles of the invention and embodying the wave propagation means of Fig. 2, and

Fig. 4 shows a modification of the input coupling means of the tube of Fig. 1.

The slow wave propagating structure illustrated in Fig. 1 comprises a series of conductive discs or fins l spaced along and supported on a conductive rod 3. Preferably the discs I are identical in thickness and diameter, equally spaced, and parallel. A hollow conductive cylinder 5, having an inner diameter greater than the diameter of the discs I, may surround the disc structure coaxially of the rod 3. However, the cylinder 5 is not essential to the operation of the device as a conductor of slowly travelling Waves.

In the operation of the device of Fig. 1, wave energy applied to one end of the assembly propagates toward the other end in a TM (transverse magnetic) mode, with a velocity which depends upon the various dimensions of the structure and also upon the frequency of the energy. The fins or'discs I act like short circuited radial line or waveguide sections, producing an effect similar to distributed loading on a transmission line.

The curves of Fig. 2 show the ratio of the velocity of propagation 'u on the disc assembly to the velocity of light 0'', as a function of the disc radius a, the internal radius 1) of the conductor 5, and the frequency f of the applied wave energy. It is assumed that the disc radius a is five-times the radius g of the rod 3, and the disc spacing z is four times the disc thickness d. These assumptions are for the purpose of examples only.

The curve I, marked represents the characteristics of a structure in which the outer conductor 5 is absent or is so remote as to have negligible effect. It is apparent that the velocity ratio may be made substantially any desired value, by proper choice of the radius a, at any particular frequency f. However, as f varies, the velocity will wary also.

When

i. e. the outer conductor is about ten percent larger than the discs, the velocity does not vary so much, with either frequency or disc radius. Thus the velocity is more nearly independent'of frequency, as shown by the curve 9. With an outer conductor only about five percent larger than the discs curve i I) the effects of disc radius and frequency on the velocity are still less, and the velocity ratio is also less than it was with the somewhat larger outer conductor.

Fig. 3 shows a travelling wave amplifier tube incorporating a structure like that of Fig. l. The discs i are supported on the rod 3 which extends lengthwise of the tube. The outer conductor 5 is omitted. An annular electron gun is provided, including a ring shaped cathode l3 and a heater l5. The heater i5 is connected to terminal rods I1 and I9 which extend through seals 2! and serve to support the cathode assembly. An annular focussing electrode 23 surrounds and is connected to the cathode l3.

The electron gun is enclosed by a conductive body 25 formed with an annular cavity 21 containing the electrode assembly and provided with an annular slit 29 from which the electron stream is to emerge. The body 25 is provided with a central hole 3!. A coaxial inner conductor 33 extends through the hole 3| and terminates at its left hand end in a fitting 35 adapted to engage the inner conductor of a coaxial input line, not shown. A sleeve 3? extends from the body 25 to engage the outer conductor of the input line. A seal 39 is provided between the

members

33 and 31.

The hole iii in the body 25 merges at its right hand end with a conical opening ii, whose diameter at the base of the cone is somewhat less than the diameter of the discs l. The inner conductor 33 is connected to the small end of a conical body Q3 inside the opening 4| The inner surface of the opening 4! and the conical body 43 cooperate to act as a transmission line section which is tapered in cross section but not necessarily in impedance, and functions as a transformer for matching the line 3|, 3-3 to the disc assembly. The conical line 4!, 43 may have a length of approximately one quarter wavelength at the mean frequency of operation of the tube.

A vacuum-tight envelope 45, which may be made of glass, extends over the disc-.on-rod structure from the conductive body 25 to another conductive body 41 at the output end of the tube. The body 47 includes a wall which forms part of the vacuum enclosure and functions as a collecting electrode. A coaxial line transformer ll, 43, like the transformer ii, 43, matches the disc structure to a line section 3k, 33' which is provided with terminals 35 4 and 31 for engagement with an output line, not shown.

In the operation of the described tube, the cathode 33 and focussing electrode 23 are maintained at a highly negative potential with respect to the rest of the tube. The walls of the annular slit 29, being positive with respect to the cathode, produce a field which accelerates the electrons. The resulting electron stream is in the form of a hollow cylindrical tube whose inner diameter is slightly larger than the discs l, and flows the length of the disc assembly to the wall 49, where it is collected.

Input wave energy applied to the line 3!, 33 sets up a wave which travels on the disc structure toward the output end of the tube. The accelerating voltage, i. e. the negative potential of the cathode, is adjusted so that the electron velocity is about the same as the wave velocity. Electrons in a negative-gradient portion of the field of the wave are retarded, while those in a positive gradient portion are accelerated.

As the stream travels down the tube, the electrons tend to form longitudinally separated bunches, and the electron density varies. This variation in density of the moving stream produces a field like that which caused it, thus reinforcing the wave as it travels along the tube. At the output end, the amplified wave appears at the terminals 37' for application to any suitable utilization device or load.

Typical dimensions of a tube like that of Fig. 3, designed for operation at 9060 megacycles per second, are as follows:

.' Disc radius a inch .343 Rod radius 9 do .103 Disc spacing z do .0383 Disc thickness d do .0113 Rod length do 6.3 Accelerating voltage volts 2180 Total cathode current ma 100 The foregoing data are given by way of example only.

Although a specific embodiment of the invention has been described, it will be aparent from the description that many variations are possible. For example, as shown in Fig. 1 and discussed in connection with Fig. 2, an outer conductor may be provided around the disc assembly. Magnetic focussing means, such as are used with travelling wave tubes of the helix type, may be provided, although the necessity is not as great with tubes of the present type because the beam density may be less.

The structure of Fig. 3 is not a particularly broad band device, since its propagation characteristic is like that depicted by the curve 1 of Fig. 2; however, by using an outer conductor and a somewhat higher accelerating voltage, a characteristic like that of the curve 9 or the curve ll may be obtained, with useful amplification available throughout a relatively wide band.

The impedance presented by the disc-on-rod structure to the input and output coupling means will depend upon the radial distance r from the axis (see Fig. 2), at which the connection is made. If the input or output line terminates near the root of the fin or disc, 1" is approximately equal to g and the impedance presented to the line will be of the order of one quarter ohm in a typical case. Since usual coaxial transmission lines have impedances of the order of50 ohms, this represents an extreme mismatch.

As r is increased, the impedance will increase.

However, as r approaches the disc radius a, the impedance also becomes increasingly dependent on frequency. Thus, in order to provide a reasonably good impedance match over an appreciable frequency band, it is necessary to make 1" substantially less than a, and use transformers between the disc assembly and the input and output lines.

In the device of Fig. 3, the conical line sections 4|, 43 and 4|, 43 are designed as quarter wave transformers, having a characteristic impedance which is the geometric mean of the impedance of the coaxial lines and the impedance of the disc assembly at the selected radius r. This arrangement is satisfactory for a tube like that of Fig. 3, which is designed for relatively narrow band operation. However, in a tube including the outer sheath 5 and designed for broad band use, such a matching system imposes an undesirable limitation on bandwidth, because the

line

41, 43 will be too much longer or shorter than one quarter wavelength at frequencies considerably higher or lower than the design center frequency.

The desired broad-band impedance transformation may be obtained by using, instead of the quarter wave uniform impedance line 4|, 43, a line section whose characteristic impedance varies smoothly from that of the

line

31, 33 at one end to that presented by the disc assembly at the other end. However, such a line section must be considerably longer than one quarter wavelength, and preferably several wavelengths long.

Fig. 4 shows a modified input coupling arrangement wherein impedance matching is achieved at least in part by tapering the central rod 3 of the disc on rod assembly, making the radius g of the rod approach the disc radius a in the vicinity of the end of the structure. The radial depth of the spaces between adjacent discs thus becomes less and less, so that the structure is substantially a solid rod 4 of radius a. at its end. The input line 3|, 33 expands through a conical line section 4|, 43" to the size of the line formed by the solid rod 4 and the sheath 5. The conical line section 4|, 43 may be designed simply to function as an. adapter between the lines 3!, 33 and 4, 5, or may be designed to provide also a certain amount of transformer action as described above.

The annular slot 29, through which the electron stream is projected, is preferably made about one quarter wavelength long at the center of the frequency band through which the tube is to operate. This presents an effective short circuit at the end of gap 30, preventing leakage of input energy through the slot 29.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electron discharge device comprising a conductive rod, a plurality of parallel discs on said rod spaced at intervals substantially less than their diameter, means for producing a tubular electron stream, the inside diameter of said stream being approximately equal to the diameter of said discs, and directing said stream along said rod and coaxially thereof, means for applying wave'energy to one end of said disc and rod assembly to induce propagation of said energy along said assembly in the direction of flow of electrons in said stream, and means for extracting wave energy from the other end of said disc and rod assembly.

2. A travelling wave tube structure including a rod, a plurality of spaced parallel discs on said rod, a coaxial transmission line, a conical conductive member with its smaller end connected tothe inner conductor of said line and its larger end connected to the end of said rod, the diameter of said larger end being greater than that of said rod, but less than the diameter of said discs, and a second conductive member including a conical cavity surrounding said conical conductive member, said second member being connected to the outer conductor of said line.

3. A travelling wave tube or the like including a wave propagating structure in the form of a rod and a plurality of spaced parallel discs on said rod, means for coupling a coaxial transmission line to said structure comprising a conical outer conductor connected to the outer conductor of said line and forming an outwardly flared extension thereof at least as far as the adjacent end of said rod, an inner conductor including a conical portion connected to the inner conductor of said line and a cylindrical portion extending from the base of said conical portion to said end of said rod, and a series of axially spaced circumferential slots in said cylindrical portion, the depths of said slots increasing from a minimum in the vicinity of the point of connection to said base of said conical portion to a maximum in the vicinity of the point of connection to said rod.

4. The invention set forth in claim 1, further including an outer tubular conductor coaxially surrounding said disc and rod assembly and having an inner diameter larger than the diameter of said discs.

5. The invention set forth in claim 4, wherein the inner diameter of said outer tubular conductor is more than about five percent larger than the diameter of said discs.

LESTER M." FIELD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,064,469 Haefi' Dec. 15, 1936 2,122,538 Potter July 5, 1938 2,294,881 Alford Sept. 8, 1942 2,338,441 Kohl Jan. 4, 1944 2,395,560 Llewellyn -1- Feb. 26, 1946 2,438,913 Hansen Apr. 6, 1948 2,479,687 Linder Aug. 23, 1949 2,511,407 Kleen et al June 13, 1950 2,516,944 Barnett, Aug. 1, 1950 2,566,087 Lerbs Aug. 28, 1951 2,567,718 Larson Sept. 11, 1951 2,567,748 White Sept. 11, 1951 OTHER REFERENCES The Theory of Disc-Loaded Wave Guides, Journal of Applied Physics, vol 18, p. 996-1008, Nov. 1947.

Article of Warnecke and Guenard, pp. 272-278, incls., Annales de Radioelectricite, vol. 3, Nov. 14, Oct. 1948.

Article by Doehler and Kleen, pp. 117-118, Annales de Radioelectricite, vol. 4, No. 16, Apr. 1949.